Electrographic printers and copiers utilizing developer comprising toner, carrier, and other components use a developer mixing apparatus and related processes for mixing the developer and toner used during the printing process. The term “electrographic printer,” is intended to encompass electrophotographic printers and copiers that employ dry toner developed on an electrophotographic receiver element, as well as ionographic printers and copiers that do not rely upon an electrophotographic receiver. The electrographic apparatus often incorporates an electromagnetic brush station or similar development station, to develop the toner to a substrate (an imaging/photoconductive member bearing a latent image), after which the applied toner is transferred onto a sheet and fused thereon.
A toner image may be formed on a photoconductor by the sequential steps of uniformly charging a surface of the photoconductor in a charging station using a corona charger, exposing the charged photoconductor to a pattern of light in an exposure station to form a latent electrostatic image, and toning the latent electrostatic image in a development station to form a toner image on the photoconductor surface. The toner image may then be transferred in a transfer station directly to a receiver, e.g., a paper sheet, or it may first be transferred to an intermediate transfer member or ITM and subsequently transferred to the receiver. The toned receiver is then moved to a fusing station where the toner image is fused to the receiver by heat and/or pressure.
In electrostatographic copiers and printers, pigmented thermoplastic particles, commonly known as “toner,” are applied to latent electrostatic images to render such images visible. Often, the toner particles are mixed with and carried by somewhat larger particles of magnetic material. During the mixing process, the magnetic carrier particles serve to triboelectrically charge the toner particles to a polarity opposite that of the latent electrostatic image. In use, a developer mix is advanced, typically by magnetic forces, from a sump to a position in which it contacts the latent electrostatic image.
The relatively strong electrostatic forces associated with the latent electrostatic image operate to strip the toner from the carrier, causing the toner to remain with the latent electrostatic image. Thus, it will be appreciated that, as multiple latent electrostatic images are developed in this manner, toner particles are continuously depleted from the developer mix and a fresh supply of toner must be dispensed from time-to-time in order to maintain a desired image density. Usually, the fresh toner is supplied from a toner supply bottle mounted upside-down, i.e., with its mouth facing downward, at one end of an image-development apparatus. Under the force of gravity, toner accumulates at the bottle mouth, and a metering device, positioned adjacent the bottle mouth, operates to meter sufficient toner to the developer mix to compensate for the toner lost as a result of image development. Usually, the metering device operates under the control of a toner concentration monitor that continuously senses the ratio of toner to carrier particles in the developer mix.
Development stations require replenishment of toner into a developer sump to replace toner that is deposited on the photoconductor or receiver as well as a magnetic carrier that are mixed together uniformly to form an effective developer. The developer must be mixed and transported to a position where it can be in contact with the latent electrostatic image. If the photoconductor picks up too much magnetic carrier the printing process is compromised. This can lead to many problems from poor prints to no prints at all as well as a depletion of magnetic carrier to a point where an image is not effectively formed. As a feed apparatus picks up developer from a feed roller the amount of developer left near the rear portion of the feed roller is greatly decreased to the point where there is no developer left to transport to the latent electrostatic image and printing stops. This is not an easy problem to solve since a simple change in developer amount or charge can quickly change conditions near the photoconductor. This problem is enhanced when there is less developer left in a feed channel, then a pick-up point becomes even further from the feed roller, and since the magnetic force is decreased by multiples as the distance decreases, this makes the problem quite significant. This appears to become enhanced and complicated at higher print speeds.
The present invention corrects the problem of magnetic carrier transport from the photoconductor surface back to the development station. For example with a two-component development system, marking media (dry ink) is electrostatically adhered to magnetic particles of ferrite (carrier) in the development station. The toner, such as dry ink, is deposited to image areas on the photoconductor, while the carrier returns to the development station, where it can then be repopulated with dry ink to continue the electrophotographic cycle. During the development process, carrier can also deposit on the photoconductor surface, and cannot return to the development station without some intervention. The apparatus and related methods described allow the printer to produce the high quality prints or powder coatings required by consumer demand by removing magnetic carrier in areas on the photoconductor that will interfere with the image formation and operation of the printer.